In recent years, assay methods utilizing immune responses, such as enzyme immunoassay methods and chemiluminescent enzyme immunoassay methods, have been widely used in the fields of clinical examinations and diagnostic pharmaceuticals. A system for such a biochemical assay method (hereinafter referred to simply as an assay system) is required to be capable of specifically and accurately detecting a protein derived from various organisms. The protein is used as the assay target and has to remain stable for a long period of time. However, most of proteins are readily denatured or deactivated to lose the bioactivities due to various factors such as temperatures, lights, pHs, salt concentrations, and oxidation factors. Thus, during the storage of the protein, it is important to protect the protein from the external factors to maintain the bioactivity.
Meanwhile, in a commonly known method for retaining an antibody activity, antigenicity, enzymatic activity, or the like of a clinical diagnosis antibody or antigen, a labeled antibody or antigen, or the like in a solution state, frozen state, or freeze-dried state, an additive such as a sucrose, saccharose, or bovine serum albumin (hereinafter referred to as BSA) is added thereto (Non-Patent Literature 1). Furthermore, a method using a particular synthetic polymer for stabilizing a protein is disclosed in Patent Literature 1.